The present invention relates to a method and apparatus for image subtraction in order to extract non-common information from two light images.
The observation of two images in order to emphasize the difference between them is of extreme importance in many fields, e.g., in the medical field for comparison of X-ray transparencies, in aerial photography for comparison of images, and in industry for detection of defects, a good example being the microelectronic mask industry.
Many techniques for the extraction of the difference information have been proposed. Some of these techniques are based on coherent illumination (lasers); others require the generation of an intermediate step (usually a photographic record) and therefore cannot be performed in real time; and others are limited to transparencies as input functions.
One known technique is described in the article titled "Real Time Grid Coding and Interlacing by Image Subtraction" by N. Konforti and E. Marom in Applications of Holography and Optical Data Processing, Pergamon Press (1977), pages 85-91. The principle behind the real time coding according to this technique is based on the projection of the two images that are to be compared onto a common plane in perfect registration. However, the optical system is such that the images are projected onto the common plane from slightly different orientations. By positioning a grating, e.g., a Ronchi ruling grid, just in front of this common plane, the shadow of the grating provides the necessary coding such that the two images are interlaced on the common image plane, with the non-common information being modulated by the grating frequency and the common information being modulated by double the grating frequency or a higher-multiple thereof. Thus, by the use of known filtering techniques, the common information may be suppressed and only the non-common information may be outputted for further processing or for further display. The system described in the above-cited article provides real time coding and interlacing by the use of a beam-splitter which projects the two images simultaneously from slightly different angles onto a photographic plate via the Ronchi ruling grid spaced a short distance in front of a photographic plate such that the shadow of the grid interlaces the two light images impinging the photographic plate. However, the filtering operation described in that article cannot be performed in a real time manner. Further details as to the construction and operation of such a system are found in the above-cited article, which is herein incorporated by reference.